Sunday, July 16, 2006

Youth was easy for big predatory dinosaurs - but adulthood and old age much harder to survive, a mass graveyard of tyrannosaur fossils suggests.

This first study of dinosaur population distributions shows that most juvenile tyrannosaurs survived to reach sexual maturity, but then their death rate increased sharply in adulthood. This life-pattern is similar to those of long-lived birds and mammals.

Palaeontologists have long wondered why fossils of juvenile dinosaurs are few and far between. The answer is 'they just didn't die at those ages', says Gregory Erickson of Florida State University in Tallahassee, US, who led the new study. 'Not until they reached mid-life did they start getting knocked off.' He says the pattern echoes that of large modern herbivores like elephants and cape buffalos.

Biologists study population distributions of modern animals by counting individuals and keeping track of deaths. This is not possible for extinct creatures, and fossilised remains are also scant for many dinosaurs. But Erickson turned to the tyrannosaur family that roamed North America from about 80 to 65 million years ago, late in the Cretaceous period, which left abundant fossils.